Applying latent heat of condensation for deceleration of free fatty acid formation in brown rice by humidified hot air fluidized bed dryer

Abstract

Latent heat of condensation is an energy-releasing reaction when water vapor comes in contact with the surface of an object containing temperature lower than the dew point or saturation temperature of a drying media. During the condensation, the thermal energy in the system is suddenly absorbed by the lower temperature object in order to make the phase change. Subsequently, the object temperature rapidly increases to the mentioned temperature of the drying media. In this research work, the energy-releasing property from condensation was applied to paddy during fluidizing process to prevent the rancidity, caused by hydrolysis process of lipids in rice bran by lipase enzyme into free fatty acid (FFA), of the brown rice during storage. Paddy with post harvested moisture content of 33.3% dry basis was dried out in varous conditions under humidified hot air (HHA) condition generated of saturated water vapor by electrical boiler.  The hot water vapor was injected into drying systems of fluidized bed dryer to blend with the hot air (HA) which was set in different drying temperature of 100-150^oC and 1-30% of relative humidity (RH). The drying kinetic result revealed that the condensing reaction occurred at the initial stages of drying process when the paddy temperature was lower than the dew point temperature of HHA media. During the condensation process carried on, the grain temperature rapidly increased to the higher level than that under the normal drying process with HA.  Its increase, moreover, was depended on increasing RH of the HA. The occurrence of condensation resulted in better deceleration of FFA content of the brown rice compared to drying by HA during storage for 6 months (analyzed every month).